The trend in the printer industry is to make higher resolution images at a faster rate. To do this, printer manufacturers are striving to produce prints with more dots per inch, and to develop a better understanding of dot mixing and color matching. In the case of ink jet printers, a lot of the control comes from the type of spray port which delivers ink to the receiving medium. The spray ports are extremely small holes through which the ink is forced out and onto the paper. The printer manufacturers can alter the type and number of spray ports. Typical ink jet cartridges may have from approximately 30 to 200 spray ports, and the correct operation of the spray port is critical to the proper operation of the printer. It is therefore important to filter out contamination or agglomerations which may be present in the ink prior to the ink reaching the spray ports.
Ink can be forced out the spray ports using a number of different technologies. The ink can be pressed out by a piezoelectric element which expands with a voltage and compresses the ink, creating a pressure to force the ink from a small reservoir. Other methods for forcing the ink through the spray port are referred to as bubble jet and thermal jet techniques. These and other related ink jet printing technologies will hereafter be referred to as ink jet printers, and the cartridges or housings into which the filters are placed will be referred to as "ink cartridges." There are a number of other ways of transferring ink, dye, or wax to a printing medium. Some of these technologies use heat to transform a solid wax or dye and prepare it for transfer. Other technologies directly sublime the solid to a vapor prior to transfer, which are sometimes referred to as wax thermal, dye thermal, wax/dye thermal, direct wax, direct dye, and phase change technologies. For convenience, these as well as the ink jet printing technologies described above will hereafter be referred to as "ink printers." In addition, ink, dyes, wax, and other similar combinations and types of image producing material will be referred to for convenience as "ink".
In all of these ink printer systems, it is important to ensure clean delivery of the ink. If contamination clogs the spray ports, the operation of the ink cartridge is hindered. The flow of ink to the paper may be reduced and/or the plugged ports may drip.
The trend in the industry is to make the diameter of the spray ports even smaller to improve the resolution of the image produced. It has therefore become increasingly more important and difficult to filter out particles which may plug these smaller spray ports.
The most commonly used filter medium is a woven stainless steel screen. These screens can be made with a number of different strands per inch in order to create a pore size for filtration of particles larger than a predetermined size. For example, a screen having 250.times.1400 strands per inch (98.times.550 strands per cm) in a double Dutch twill weave, as available from Tetko Inc. will provide filtration for 19 micron nominal diameter and larger particles. The efficiency of these screens will be discussed later herein.
The screens used in these applications are typically stainless steel to ensure chemical compatibility with the ink. In most cases, the ink contains surfactants and/or solvents, as well as other compounds, to promote wetting of the paper or printing substrate. Furthermore, the inks may be acidic or basic.
A significant difficulty encountered with woven screens is that they provide very little open area for filtration. The interstices between the fibers create the flow channels for the fluid, but this area is typically only 10 to 20% of the overall area of the filter. Thus, the small available area for filtration creates a high resistance to flow for the ink. In addition, the ink cartridges are being required to dispense the ink at a rapid rate because of the demands associated with higher speed printers and the increased use of color printing. Color images have a much higher degree of ink coverage to create an image, and therefore, require more ink to be dispensed. These new trends make lowering the resistance to flow more critical. For the reasons noted above, woven screens are not the ideal filter medium for the filtration of particles at higher flow rates.
In addition, the dimensions of woven screens are limited by the number of strands per inch that can be woven, and the screens become increasingly more expensive as the number of strands per inch increases. Thus, cost of woven screens limits their use in these high volume, cost sensitive products.
A further problem with stainless steel screens is that they are difficult to bond and seal to the , typically, plastic ink jet cartridge housings. The filter material is typically heat staked to the plastic, and because of the irregular edges of the screen, a complete seal is difficult to produce. When cut into disc shapes, the woven screen has ragged edges which if not sealed properly can create a leak path for large particles to pass through. In some cases, the stainless steel screen is applied with an adhesive to ensure a good seal. However, this is a time consuming and costly process. Thus, yield rates for applying these screens to the ink cartridge housing are below a desirable level due to these processing problems.
Further, the stainless steel screen can shed loose particles or fibers which can then contaminate or clog the spray ports. When the screen is cut, typically by die cutting, the overlapping metal strands can be pinched and broken. These small screen fragments can shed after the filter disc is adhered to the ink jet cartridge. When one of the shed strands gets downstream of the filter disc, it can clog the spray port head, creating problems with the printer.
Finally, in some applications, such as thermal dye sublimation, a solid wax is heated until it is a fluid. The fluid is then filtered prior to developing the image. It is important to ensure that particles, such as contaminants or larger non-fluidized pieces of the wax do not clog the system. Therefore, a filter with high temperature stability may be required. Temperatures of 100 to 150.degree. C. are common. In addition, in thermal ink jet printers, as well as the other ink jet printer technologies, the ink may be heated in the area of the spray port. Again, it is important to have a filter material that can withstand these elevated temperatures.
Accordingly, it is a primary purpose of the present invention to provide an improved filter for filtering contaminates from ink within an ink printer cartridge. Such improved filters preferably have a high percent open area for filtration and, therefore, provide a low resistance to flow. In addition, the required filtration efficiency of the printer is met.
A further purpose of the present invention is to provide an ink filter which is chemically inert, is resistant to elevated temperatures, and is easily bonded to the materials commonly used in ink printer cartridges.
These and other purposes of the present invention will be apparent based upon a review of the following specification.